1. Field of the Invention
The present invention generally relates to a motor control system for controlling rotational speeds and phases of a motor. More specifically, the present invention is directed to a motor control system suitable to control a capstan motor and a drum motor of a magnetic recording/reproducing apparatus.
2. Description of the Related Art
Conventionally, rotational speeds and phases of motors are controlled. In particular, very precise servo controls are required for motors employed in a VTR (video tape recorder) and the like.
In FIG. 6, there is shown one example of such a motor control apparatus for a VTR.
In this drawing, reference numeral 41 indicates a phase detector for detecting a phase difference between a reference signal FR1 produced from the vertical sync (synchronization) signal and an output signal of a CTL amplifier 42. Reference numeral 42 refers a CTL amplifier for amplifying a control signal reproduced by a control head, reference numeral 43 is an adder for adding an output "ep" of the phase detector 41 to an output "ev" of a speed detector 47, and reference numeral 44 denotes a drive amplifier for producing a drive signal to drive a motor 45 in response to an output "eo" of the adder 43. Furthermore, reference numeral 45 is a capstan motor for rotatively driving the capstan, reference numeral 46 indicates a frequency generator (will be referred to an "FG" hereinafter) for sensing the rotational speeds of the capstan motor 45, and reference numeral 47 shows a speed detector for outputting the voltage signal "ev" in correspondence with the rotational speed of the capstan motor 45 in response to the output from the FG 46. Also, reference numeral 48 indicates a tape transported by the capstan at a predetermined speed, and reference numeral 49 denotes a control head for reproducing the control signal recorded on a control track of the tape 48.
In the motor control apparatus, the reference signal FR 1 produced from the vertical sync signal is supplied to the phase detector 41 to detect the phase difference between this reference signal FR1 and the control signal amplified in the CTL amplifier 42, and then the voltage signal "ep" responding to this phase difference is outputted.
From the output of the FG 46 for detecting the rotational speeds of the capstan motor 45, the speed detector 47 outputs the voltage signal ev in accordance with the rotational speed of the capstan motor 45.
Both the output "ep" of the phase detector 41 and the output "ev" of the speed detector 47 are supplied to the adder 43 so as to be added with each other, thereby producing the voltage signal "eo".
The voltage signal "eo" of the adder 43 is applied to the drive amplifier 44 to produce the drive signal by which the capstan motor 45 is driven and controlled.
Assuming now that the rotational speed of the motor 45 is lowered, the frequency of the output signal from the FG 46 is lowered, so that the output "ev" of the speed detector 47 is lowered. Then, since this output "ev" is inverted and added in the adder 43, the output "eo" of the adder 43 is increased, so that the capstan motor 45 is controlled by the drive amplifier 44 in such a manner that the rotational speed of the capstan motor 45 is increased. As a result, it is so controlled that the rotational speed of the capstan motor 45 becomes constant.
When the tracking is shifted, the phase of the control signal reproduced by the control head 49 is varied, and then the phase difference between the reference signal FR1 and the control signal f.sub.CTL becomes large. As a consequence, since the voltage signal "ep" in accordance with this phase difference is outputted from the phase detector 41, the voltage signal "eo" outputted from the adder 43 is similarly changed, and the transportation of the tape 48 by the capstan is controlled by the capstan motor 45. Therefore, the capstan is controlled by the capstan motor 45 in such a manner that the tracking of the reproducing head is continuously maintained.
In the motor control apparatus shown in FIG. 6, after the capstan motor 45 is once locked, a very stable control can be performed. However, this motor control apparatus has the following drawbacks.
That is, the adding result between the voltage signal "ep" corresponding to the phase error signal derived from the phase detector 41 and the output signal "ev" corresponding to the speed error signal derived from the speed detector 47 is fed back, and the capstan motor 45 is controlled by way of the double servo loop. The ratio of this voltage output "ep" to the output "ev" is determined by experiments, taking account of stabilities of the servo system.
However, when this adding ratio would be too small, or too large, the output "ep" would interfere with the output "ev" during the transition of the phase pull-in operation such as the rising operation of the capstan motor, resulting in unstable phase pull-in operations, or uncertain lock-in time.
When the double servo loop would be constructed of a microcomputer (referred to as "CPU") control in the motor control apparatus shown in FIG. 6, since the data process specific to the speed control system should be performed at high speeds, heavy loads would be given to this CPU for controlling the motor.
In particular, generally speaking as a CPU (microcomputer) mounted on a VTR is so arranged as to perform not only a motor control, but also a malfunction diagnosis, a system control, a measurement of servo data, and so on, a lengthy time is required to execute these process operations by the CPU. Nevertheless, most calculation processing time would be consumed only to control the motor, and thus, there is less time to process other CPU works. Accordingly, another problem may occur in that the operation efficiency of the CPU would be lowered.
To solve this problem, a high-speed CPU may be employed. However, since such a high-speed CPU has a high cost, there is another problem that the manufacturing cost of a product would be increased when such a high-speed CPU would be used.